Case Study:

Low Impact Development

On undeveloped land, trees, forests, and fields act as natural buffers that capture rainfall and snowmelt, filter pollutants, and slow down stormwater runoff to enable it to infiltrate into the ground. When land is converted from this natural state and replaced by impervious roadways, rooftops, and parking lots, this buffering ability is lost. More polluted stormwater runoff reaches our waterways more quickly, bringing nitrogen, phosphorous, sediment, toxics, and bacteria with it—and causing flooding, erosion, and channelization along the way. Low Impact Development (LID) techniques aim to maintain the hydrologic cycle and protect water quality throughout the development process.

The Village at Springbrook Farms is a good example of what is possible when a township that takes a long term view works with a developer willing to innovate. Recognizing the problems associated with detention basins and the need to sustain groundwater supplies, South Londonderry Township requires infiltration wherever possible on new development sites. Prior to development, Springbrook Farms comprised a gently rolling landscape under cultivation for soybeans and corn. Most of the northern side of the site drained into closed depressions, while the southern side drained into surface channels during storm events.

Rather than use the mammoth detention basin that was called for in the original development plan, engineers from Cahill Associates studied the site's natural conditions and created a stormwater management system that mimicked the existing regime. The overall approach was to keep the stormwater as close to the source as possible, cleansing and recycling it with a variety of BMPs (Best Management Practices). Porous asphalt pavement was used extensively—for sidewalks, paths, and parking areas—and typically had stone-filled recharge beds built underneath to purify runoff before enabling it to seep back into the ground. Infiltration beds were also used underneath non-porous driveways, while rain gardens, vegetated swales, and other landscape features were incorporated throughout the site to mitigate any potential impacts from runoff.

It's always best if you can to keep the water on site and percolate it down through the ground right where it falls. As time goes on, people will come to desire these types of communities and we hope to be able to use that as a marketing feature here.-Scott Campbell, Owner Brownstone Real Estate Company.

After an extensive site investigation, soil and infiltration tests took place. It was determined that the "pre-development" stormwater runoff condition was essentially zero for both rate and volume of runoff, because the many closed depressions acted as small catchments. It was agreed that rather than "over-engineer" to create a costly and ineffectual collection and conveyance system, the site would be treated as though runoff could occur during extreme weather events. For volume control, the various BMPs were designed so there would be no increase in stormwater or rainwater runoff volume for the 2-year, 24-hour storm. For peak rate control and to prevent localized flooding, the various BMPs were interconnected with a shallow piping system capable of conveying 2-year through 100-year flows without overtopping the BMPs or creating damaging flooding.

In the end, more than 100 storage/infiltration BMPs have been distributed throughout the site. Each was located and sized according to its drainage area, and accounts for both storage volume and the amount of surface area required to "spread out the water" to avoid over-concentrating infiltration. With all elements working together, Springbrook’s stormwater management system treats pollutants, re-charges the groundwater, and maintains the water table, and provides flood control, while preventing destructive effects downstream.

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This excerpt talks about the importance of properly managing runoff to make a development project succeed.